Television apparatus



Sept. 19, 1939. ,A. v. BEDFORD 2,173,467 I TELEVISION APPARATUS Filed Oct. 31, 1935 4 Sheets-Sheet 1 WQEEEMQ hm,

p 19, 1939. A. v. BEDFORD 2,173,467

TELEVIS ION APPARATUS Filed Oct. 31, 1935 4 Sheets-Sheet 2 1' l TIME T1 TIME Au dy p 19, 1939. A. v. BEDFORD 2,173,467

TELEVIS ION APPARATUS Filed Oct. 31, 1935 4 Sheets-Sheet 3 Sept! 1939. A. v. BEDFORD 2,173,467

TELEVIS ION APPARATUS Filed Oct. 31, 1935 4 sheetsvsheet 4 iiwussscs awbjfiz md A jw E? Patented Sept. 19, 1939 UNHTED STATES mam TELEVISION APPARATUS Alda V. Bedford, Collingswood, N. J., assignor to Radio Gorporation of America, a corporation of Delaware Application October 31, 1935, Serial No. 7,564

9 Claims.

My invention relates to cathode ray tube de fiecting circuits and particularly to circuits designed to supply saw-tooth waves to the cathode ray tube deflecting plates or deflecting coils.

This application is a continuation in part of application Serial No. 755,304, filed November 30, 1934, and assigned to the Radio Corporation, of America.

When deflecting plates are employed for defleeting a cathode ray, it is the usual practice to supply a saw-tooth wave to the deflecting plates through a power tube and a transformer. It has been found that in such a circuit there is distortion of a saw-tooth wave such that if a saw-tooth wave of perfect wave form is supplied to the power tube it appears across the deflecting plates as a saw-tooth wave which is bent over, that is, the main portion of the saw-tooth is curved instead of straight.

The same difiiculty is encountered when deflecting coils are employed, this being true whether the deflecting coils are coupled to the output tube through a transformer or through impedance coupling comprising a\choke coil and :5 coupling condenser.

One object of my invention is to provide an improved cathode ray tube deflecting circuit which substantially overcomes the above-mentioned difficulty.

:0 More specifically, an object offmy invention is to provide an improved deflecting circuit which supplies waves of substantially perfect saw-toothwave form to the deflecting plates or coils of a cathode ray tube.

.5 A still further object of my invention is to provide a circuit which supplies to cathode ray tube deflecting plates or coils waves which have a substantially perfect saw-tooth wave form and which are suitably modified to provide correc- 0 tion for the so-called keystone efi'ect obtained in cathode ray tubes having an inclined screen.

In a preferred embodiment of my invention I employ a capacitor-resistor network for pre distorting a saw-tooth wave before it is im- 5 pressed upon the input circuit of the power tube.

This network is so adjusted that it acts preferentially upon the low frequency components of the saw-tooth wave, this preferential action accentuating the amplitude of the low frequency com- 0 ponents and also producing a shift in their phase.

In one particular embodiment of my invention, a capacitor-resistor network is employed which filters out from the horizontal deflecting 5 waves all low frequency components which have been added as a result of the keystone correction. This same network also pre-distorts'the' horizontal deflecting waves to compensate for distortion unavoidably produced in the output circuit of the power tube.

Other objects, features and advantages of my invention will appear from the following description taken in connection with the accompanying drawings, in which Figure 1 is a circuit diagram of a television. transmitter embodying my invention,

Fig. 2 is a simplified plan view of the cathoderay transmitter tube shown in Fig. 1,

' Fig. 3 is a similar side view of the above-mentioned cathode-ray tube,

Fig. 4 is a circuit diagram showing a modification of a portion of the circuit shown in Fig. 1,

Figs. 5 to re curves which are referred to in explaining the keystone correction feature of my invention, 1

Figs. 11 and 12 are curves which are referred to in explaining the vertical deflection correcting feature of my invention,

Fig. 13 is a circuit diagram showing a modiflcation of another portion of the circuit shown in Fig. 1, and

Figs. 14 and 15 are curves which are referred to in explaining the operationof the saw-tooth pre-distorting network.

Referring to Fig. 1, the television transmitter includes a cathode-ray transmitter tube I of the type which is described in an article by V. K. Zworykin, published in the January, 1934, issue of the Proceedings of the Institute of Radio Engineers. This tube includes an electron gun consisting of a cathode 3, a control electrode 5 and a first anode '7. A second anode 9 is provided which, in addition to accelerating the electrons, aids in the electrostatic focusing of the electron beam. The second anode also functions as a collector of photo-electrons which are emitted from a mosaic ll of light-sensitive elements.

The light-sensitive elements of the mosaic 40 H are insulated from each other and from a metallic back plate, as described in the abovementioned publication. 4

An optical image of the object l3 to be transmitted is projected upon the mosaic H by means of a suitablepoptical system l5. In order that the light-sensitive surface of the mosaic H .may have the optical image projected thereon and,

at the same time, be scanned by the electron beam, it is positioned at an acute angle to the longitudinal axis of the cathode-ray tube. More specifically, it is positioned at an acute angle to the axis of the electron gun.

The cathode-ray tube l is provided with suitable deflecting devices, such as deflecting plates I! and deflecting coils IQ, for deflecting the electron beam both horizontally and vertically to scan the mosaic II. If an optical image is projected upon the mosaic, picture signals are developed during the scanning of the mosaic by no the electron beam, these picture signals appearing across a resistor 2| which is connected between the mosaic back plate and ground. These picture signals are supplied to a suitable transmitter 23 or to a transmission line for transmission to a receiver.

Because of the angular position of the mosaic H with respect to the electron gun, certain corrections must be made in' both the horizontal and vertical deflection of the electron beam. That such corrections are necessary will be apparent by referring to Figs. 2 and 3, like parts in these figures and Fig. 1 being indicated bythe same reference numerals. In Fig. 2 the dotted lines 25 indicate the limits of the horizontal deflection of the electron beam. It will be evident that when the beam is striking the lower portion of themosaic II it traverses a shorter path thereon than it does when it is striking the upper edge of the mosaic, providing the angle between the limits of the horizontal deflection remains constant as the beam is deflected upwardly. As a result, the beam scans a keystone shaped area on the mosaic that is indicated by the area enclosed in dotted lines 21. One of the main objects of my invention is to so correct the horizontal deflection that a rectangular, rather than a keystone, shaped area is scanned on the mosaic.

In Fig. 3 the dotted lines indicate several positions of the electron beam as it is deflecting upwardly on the mosaic. From inspection of this diagram it will be apparent that, for a given angular upward deflection of the electron beam, it traverses a longer path on the mosaic at the top thereof than it does-at the bottom. As a result, unless a suitable correction is made, the picture appearing at the receiver will be crowded together in the lower region of the picture.

Referring now to the deflecting circuits shown in Fig. 1, the vertical deflecting circuit for supy g a saw-tooth current to the deflecting coils I9 is indicated generally at 29, while the horizontal deflecting circuit for supplying a sawtooth voltage to the deflecting plates i1 is indicated generally at 3|.

Both deflecting circuits are controlled by an impulse generator 33 which generates horizontal deflecting impulses at a comparatively high frequency and vertical deflecting impulses at a comparatively low frequency. In the particular embodiment illustrated the horizontal deflecting impulses occur at the rate of 10,290 per second, while the vertical deflecting impulses occur at the rate of 60 per second. The impulse generator 33 may be of any suitable type, one form comprising a disc driven by a synchronous motor, the disc containing openings therein through which light is projected upon a photo-electric cell. An impulse generator of this type is shown in French Patent No. 742,671, dated January 4, 1933,- and, as applied to interlaced scanning, in Ballard application Serial No. 623,325, filed July 19, 1932, and assigned to the Radio Corporation of America. While the frequencies of the deflecting impulses given above are for a system employing interlaced scanning, it will be understood that my invention is not limited to such a system.

' The vertical deflecting circuit includes an emplifier tube 35, a discharge tube 31, and an output tube 39. illustrated is of the pentode type having a oath-'- 'ode 4|, a control grid 43, a screen grid 45, a

suppressor grid 41 and a plate 49. The control grid 43 is provided with a suitable bias potential by means of the usual self-bias resistor 5| which The particular amplifier tube 35- is shunted by a by-pass condenser 53. The screen grid 45 and plate 49 are supplied with positive potentials from a suitable unidirectional plate or B voltage supply (not shown) through filter resistors 55 and 51, respectively. Suitable by-pass or filter condensers 59 and GI are provided in the screen grid and plate circuits.

The vertical deflecting impulses are impressed upon the input circuit of the amplifier tube 35 through a resistor 63 and a coupling condenser 65. v The output circuit of'the amplifier tube is coupled by means of resistance coupling to the input circuit of the discharge tube 31. The discharge tube 31 may be of the three-element type, as illustrated, comprising a cathode 61, a control grid 69 and a plate 1|. The grid leak resistor 10 has a resistance value which is high enough to bias the tube 31 substantially to cut-off during the intervals between impulses, saidimpulses causing the grid to collect electrons.

- As is well known, in order to produce a flow of current through the deflecting coils l9 which has a good saw-tooth wave form, it is necessary that the voltage impressed across the coils contain a saw-tooth component in addition to the impulse component which would be required if the deflecting coil circuit were purely inductive. This saw-tooth component is provided by charging a condenser 13 from the plate voltage supply through a highly resistive circuit which includes a filter resistor 15, a variable resistor 11 and a fixed resistor 19. This charging circuit also includes another variable resistor 8|, which has a comparatively low resistance value, for supplying the necessary voltage impulse component. The positive potential for the plate 1| of the discharge tube 31 is supplied through the filter resistor 15 and the resistors 11 and 19.

In. operation, the vertical deflecting impulses which are supplied from the impulse generator 33 are given a negative polarity whereby they appear in the input circuit of the discharge tube 31 as impulses of positive polarity. The grid 69 of the discharge tube 31 normally is sufficiently negative to substantially block plate current flow between the deflecting impulses. The-condenser 13, which will be referred to as the vertical sawtooth condenser, is charged at a substantially constant rate through the high impedance circuit 15-11-19, whereby the voltage across the condenser 13 gradually increases in value. As soon as a positive impulse appears on the control grid 69, the discharge tube 31 becomes highly conductive, whereby the condenser '13 discharges very rapidly through the resistor 8| and the discharge tube. This discharge through the resistor 8| gives the desired impulse component to the voltage wave supplied to the input circuit of the tube 39. The charge and discharge of the condenser 13 produces a saw-tooth voltage wave thereacross.

The combined saw-tooth component and impulse component are impressed through a coupling condenser 83 and a re'sistor BS uponthe input electrodes of the tube 33.

The ,output tube 39 may be of the pentode type comprising a cathode 81, a control grid 89, a screen grid 9|, a suppressor grid 93 and a plate 95. The tube is provided with the usual selfbias resistor 91 and by-pass condenser 99. The screen grid 9| is supplied with positive potential through a filter resistor |0|, while the plate 95 is supplied with positive potential through a. choke coil I 93 and a resistor I05. The choke coil I93 is provided mainly for the purpose of maintaining the amplification at a proper level for high frequency components of the wave.

' The input circuit of the outputtube 39 includes a resistor I01 and a condenser I09 connected in series, the condenser I09 being shunted by a variable resistor III. The purpose of the condenser I09 and variable resistor I01 is to control the wave shape of the current flowing through the vertical deflecting coils I9, whereby the abovementioned correction may be made for the vertical deflection. This feature of the invention will be explained in detail hereinafter. The output circuit of the output tube 39 is coupled to the deflecting coils I9 through a suitable coupling condenser II3.

Referring now to the horizontal deflecting circuit 3 I, it includes an amplifier-tube H5, a discharge tube 1, a second amplifier tube H9 and an output tube I2I. The amplifier tube II5 may be of thepentode type comprising the usual cathode, control grid, screen grid, suppressor grid and plate. It is provided with a suitable negative bias by means of a self-biasing resistor I233. The plate is supplied with a positive potential through a conductor I25 and through a plate resistor I21. The screen grid is supplied with positive potential through the conductor I25 and through a filter resistor I29, a bleeder resistor I28 being connected between the screen grid and ground.

The horizontal deflecting impulses are impressed upon the input circuit of the amplifier II5 through a resistor ISI and a coupling condenser I 33. The amplified impulses are impressed through a coupling condenser I35 upon the input circuit of the discharge tube III.

The discharge tube H1 is of the screen grid type, the tube illustrated being a pentode comprising a cathode I91, a' control grid I39, a. screen grid I5I, a suppressor grid I63 and a plate I55. The tube 1 is biased at a suitablenegatlve potential by means of a self-biasing resistor HIV. The plate IE5 is su plied with positive potential through the conductor I25, a filter resistor I69, a buffer resistor I5I, and a plate resistor I53. It will be noted that the resistors I91 and I99 are provided with by-pass condensers.

The screen grid I II is supplied with positive potential through the conductor I25 and through a resistor I55. The resistor I55, together with a resistor I51 connected between the screen grid III and ground, forms a potentiometer connected across the plate voltage supply for lowering the screen grid voltage to the desired value. The resistors I51 and I55 also function as a grid leak resistor for the screen grid.

In addition to the steady voltages supplied to the plate and screen grid through the abovedescribed circuits, saw-tooth voltages are impressed upon the plate I95 and the screen grid MI through a circuit which will be described later.

In order to produce the saw-tooth voltage which is to be applied to the deflecting plates I1,'

pentode type, comprising the usual cathode, control grid, screen grid, suppressor grid, and plate.

A positive potential is supplied to the screen grid through the conductor I25 and a filter resistor I59. A positive potential is supplied to the plate through the conductor I25, the flter resistor I53, a variable wave-shaping resistor I65, and a. plate resistor I91.

In order to permita control of the deflecting voltage wave shape, a condenser I69 is connected in series with the plate resistor I51 and ground whereby the amplitude of low frequencies is exaggerated. The extent to which the lower frequencies are amplified more than the higher frequencies is determined by the extent to which signals are shunted around the condenser I69 by the variable resistor I65.

The amplified saw-tooth wave which appears across the resistor-condenser combination I51- I69 is impressed upon the input circuit of the output tube I2I. In the embodiment illustrated, the output tube MI is of the three-electrode type comprising of a cathode IN, a control grid I13, and a plate I15. The input circuit of the tube I2I includes a resistor I11 and a resistor I19 to which the control grid I13 is connected through a variable tap for the purpose of controlling the amplitude of the deflecting voltage wave. resistors I11 and I19 are shunted by a small capacity condenser I8I for the purpose of filtering out the highest frequencies in the saw-tooth wave which might produce undesired resonant effects in the transformers I83 and I85 which couple the deflecting plates I1 to the output-circuit of the tube I2I. It will be noted that the tube IZI is provided with the desired negative bias by means of a self-biasing resistor I81, while the plate potential is supplied through a filter resistor I89 and the primary of the output transformer'I83. The output transformer couples the output circuit of the tube I2I to a cable, indicated by the dotted lines I9I, which extends from the horizontal deflecting circuit to the cathode-ray tube I. to a value low enough to make the shunting effect of the distributed capacity of the cable negligible.

The auto-transformer I85, which is located in the immediate vicinity of the cathode-ray tube I, is provided for stepping up the deflecting voltage to the high value required for deflecting the electron beam.

It will be noted that in the above-described circuits all filter resistors and self-bias resistors are provided with suitable by-pass condensers.

If the correcting network I51I69-I65 were not provided in the above-described circuit, the saw-tooth voltage appearing across the deflecting plates I1 would be distorted or bent over as indicated by the curve B in Fig. 14. In the particular horizontal deflecting circuit shown in Fig. 1, this distortion of the saw-tooth wave form is caused largely by the transformers I83 and I85,

The reason for the distortion was not fully understood for some time. Measurements showed that there was a certain loss in low frequency components, which loss would cause a saw-tooth wave to bend over". However, this losswas not great enough to account for a distortion of the magnitude encountered. It was finally discovered that the explanation was in the phase shift of the saw-tooth components at low frequencies.

The effect of phase shift can readily be seen byreferring to Fig. 14 where curves C and D show the phase relation of two sine-wave components of the perfect saw-tooth wave indicated This transformer steps down the voltage The,

fleeting plates i1 is that it not only accentuates" the low frequency components but it also shifts their phase (advances their phase) to compensate for a retardation in their phase produced in the utilization network.

My capacitor-resistor network. is equally effective when employed in a deflecting circuit having deflecting coils as will be more clearly understood from a description of the action of the network I01-I09-l Ii which is "given later. While this network will be described as being provided for the purpose of controlling the degree of bending over of a saw-tooth wave, it will be understood that in circuits where a bent saw-tooth is not desired the same network may be employed to prevent bending over.

It may be noted that the wave shape of the pre-distorted saw-tooth voltage appearing across the input of output tube I2I is substantially as shown in Fig..15. It will be seen that, as a result of both phase shift and accentuation in amplibends up tocompensate for the bending over" produced in the deflecting or utilization network.

Referring to the keystone correcting portion of the circuit, it includes an amplifier tube I93 which may be of the pentode type as illustrated. The input circuit of the tube I93 is connected acrossthe saw-tooth wave condenser 13 in the vertical deflecting circuit by means of a coupling condenser I and a connection through ground.

A positive potential is supplied to the plate of the pentode I93 from the plate supply through the conductor I25 and resistors I91 and I99 which are connected in parallel. A resistor 20I is included in the circuit to lower the plate voltage tothe desired value. Resistors I91 and I99'a re provided with a by-pass condenser 203.

It will be apparent that a substantially pure saw-tooth wave appears across the resistors I91 and I99 and that this wave is out of phase with respect to the saw-tooth wave appearing across the condenser 13, the phase having been reversed by the amplifier tube I9I. The sawtooth wave appearing across the resistors I91 and I99 is shown in Figures 8 and 10.

In accordance with my invention, this sawtooth wave is utilized for modulating the voltages applied to the plate I45 and screen grid I of the horizontal circuit discharge tube II1 whereby the amplitude of the horizontal deflec-.

' this current is added' to the steady voltage provided by the plate supply.

The screen grid I 4| of the discharge tube H1 is connected through a coupling condenser. 201 to' the plate end of the resistors I91 and I99 whereby a vertical saw-tooth voltage modulates the screen grid voltage due to the flow of vertical saw-tooth currentthrough the screen grid resistor I51.

The relative values of the plate modulating voltage and the screen grid modulating voltage may be adjusted by means of the variable tap on the resistor I91. The values of both modulating voltages may be adjusted simultaneously after the desired balance has been obtained by adjusting the value of the resistor I99 which is made variable for this purpose. It will be seen that, as the electron beam is deflected upwardly, the keystone correcting circuit causes a gradual reduction in the voltages on the plate and screen grid of the discharge tube 1 since the vertical sawtooth wave is decreasing in value as indicated in In Fig. -5 there is shown a series of charging curves for the saw-tooth condenser I59 in the horizontal deflecting circuit. The upper curve 209 shows how the voltage increases across the condenser I59 as it is charged through the resistors I49,I5I and I53 with a certain voltage applied to the condenser-resistor combination. The lower curves 2| I, 2I3, H5 and 2" show how the voltage across the condenser I59 builds up for lower voltages applied to the same condenser-resistor circuit.

The curves in Fig. 6 show how the voltage across the condenser I59 varies as the said condenser discharges through the discharge tube H1 with different voltages applied to the discharge tube screen grid MI. The steepest discharge curve 2I9 represents the case where a comparatively high positive voltage is applied to the screen grid MI. The other curves 22I, 223, 225 and 221 show how the condenser I59 discharges less rapidly as the positive voltage on the screen grid MI is decreased.

Referring to Fig. 7, there are illustrated two saw-tooth curves. The upper curve 229 is the one which would be generated with the keystone correcting circuit omitted, that is, with no modulation on the plate and screen grid of the discharge tube II1. This curve has been constructed by means of the'top curve 209 in Fig. 5 and the top curve 2I9 in Fig. 6. It will be seen that the condenser I59 charges gradually along the curve 209 during the time T1 and discharges suddenly along the curve 2I9 during the time T2. It will be understood that during the time T1 the discharge tube H1 is biased beyond cut-off, while during the time T2 to a positive synchronizing impulse is being impressed upon the control grid of the discharge tube H1 to render it highly conductive. Only a small number of horizontal saw-teeth for each vertical saw-tooth has been indicated in Figs. 7 and 8 in order to simplify the drawing.

If an attempt is made .to gradually reduce the amplitude of the horizontal saw-tooth wave by the plate voltage of the discharge tube 1 has .7

been lowered, because, for all practical purposes, this is true for tubes of the screen grid type. Since the discharge rate has remained the same, the average value of the voltage across the condenser I59 has decreased in value as indicated by the saw-tooth curve. Obviously a saw-tooth wave of this character would produce a distorted picture. Specifically, it would produce a picture which would not have vertical sides.

If a three element vacuum tube were employed in place of the screen grid tube I II, the effect illustrated by the saw-tooth curve 23I would not be so pronounced since the discharge rate of the condenser I59 would change appreciably with a change in plate voltage. For this reason, by utilizing a triode discharge tube with modulation on its plate only, a keystone correction may be obtained which is appreciably better than no correction, but the correction will be imperfect.

Fig. 9 illustrates the keystone correction that is obtained by means of the circuit illustrated in Fig. 1. The saw-tooth curve 233 illustrated in this figure has been constructed by means of the chargingcurves shown in Fig. 5-and the discharge curves shown in Fig. 6, since the charging voltage and the screen grid voltage are being reduced gradually by the saw-tooth voltage indicated in Fig. 10. It is evident that by decreasing the discharge rate of the condenser l59-at the same time that its charging rate is decreased, it is possible to maintain the average voltage across said condenser substantially constant. Considering the curves 23I and 233 in Figs. 7 and 9, re-

spectively, from a slightly different view point, the saw-tooth curve 23I is objectionable because it contains a strong vertical saw-tooth compo-.

nent whereas the saw-tooth curve 233 contains no such component. It has been found from experience that my improved keystone correcting circuit is reliable in operation and that it gives .a practically perfect correction of the keystone effect.

Referring now to the previously mentioned correcting circuit for the vertical deflection error illustrated in Fig. 3, I correct for this error by causing the vertical deflecting saw-tooth wave to bend over as will be understood by referring to Fig. 11;

Referring to Fig. 11, if it were not for the fact that the mosaic II is at an angle with respect to the electron gun, for the vertical deflection, a perfect saw-tooth current wave would be desired as indicated at 235. As previously pointed out, however, it is desired to slow down the deflection of the electron beam as it is deflected upwardly. The saw-tooth may be "bent over with a resulting slowing down of the electron beam by eliminating some of the low frequency components of the saw-tooth wave 235, the saw-tooth wave 235 being the one which appears across the sawtooth condenser 13. In the vertical deflecting circuit illustrated in Fig. 1, the coupling condenser H3 and the by-pass condenser 99 are deliberately made small enough in capacity to eliminate 'enough low frequencies for bending the saw-tooth curve over more than it is desired for the correction. The resulting saw-tooth curve is indicated at 231. The saw-tooth curve indicated at 239 is the one which is desired 'for obtaining a perfect vertical deflectioncorrection. The saw-tooth wave is raised from the position of curve 231 to the position of curve 239 by means of the correcting circuit I'II09-III, previously described, which accentuates the lower frequencies. By varying the resistor II I, the vertical defleeting saw-tooth curve may be made to occupy almost any position between the two saw-tooth curves 231 and 235.

The curves in Fig. 11 represent both the flow of current through the vertical deflecting coils and the saw-tooth component of the voltage impressed upon the input electrodes of the amplifier. The wave shape of the voltage impressed upon the input electrodes of the amplifier 39 is indicated by the curve in Fig. 12. It will be seen that this voltage includes the necessary impulse component for producing the flow of saw-tooth current through deflecting coils.

In Fig. 4 there'is illustrated another circuit for controlling the curvature of the vertical sawtooth wave. Like parts in Figs. 1 and 4 are indicated by the same reference numerals. In the circuit of Fig. 4, the condenser I09 and resistors I01 and III of Fig. 1 have been replaced by a variable grid leak resistor 2. Also, the condensers I I3 and 99 in the circuit have been given a capacity sufficient to pass the low frequency components of the saw-tooth wave efliciently. The curvature of the saw-tooth wave is controlled by adjusting the grid leak resistor 2 untilthe low frequency components are attenuated to the desired degree. I

In some cases it may be preferred to deflect the electron. beam from the edge of the mosaic farther from the electron gun to the edge of the mosaic nearer the gun, that is, from top to bottom in the cathode-ray tube illustrated. It will be apparent that, in such a case, the speed of vertical deflection should be increased as the beam approaches the bottom edge of the mosaic. This may be accomplished by so adjusting the grid leak resistor III of Fig. 1 that the sawtooth wave bends upward above the line of a perfect saw-tooth, instead of downward.

Also, if the beam is deflecteddownwardly, the amplitude of the horizontal deflecting saw-tooth must be increased as the beam approaches the bottom edge of the mosaic, this being accomplished merely by applying a vertical deflecting saw-tooth wave to the discharge tube III which is 180 degrees out of phase to that assumed in describing Fig. 1. In other words, both the charging voltage and the positive screen grid voltage are increased as the beam is deflected downwardly. The change in amplitude of the charging voltage and in the amplitude of the positive screen grid voltage is always in the same sense, that is, the charging voltage and the screen grid voltage either increase together or decrease together.

In Fig. 13 there is illustrated a modification of a portion of the horizontal deflecting circuit shown in Fig. 1. The circuit shown in Fig. 13 may replace that portion of,the circuit in Fig. 1 which is at the right of the coupling condenser It will be noted that the amplifier tubes H9 and IN in this case are replacedby three tubes 300, 30I, and 302. It will also be noted that deflecting coils 303 are employed instead of deflecting plates although, of course, deflecting plates may be employed, if preferred.

The amplifier tube 300 may be of the screen grid type, the plate 304 of which is supplied with voltage through a filter resistor 306 and a plate resistor 301. The filter resistor 306 is shunted by the usual. by-pass condenser 308.

In accordance with my invention, the output circuit of the amplifier tube 300 is coupled to the input circuit of the next amplifier tube 30I through a capacitor-resistornetwork, indicated generally at 309, this network serving the double purpose of filtering out any vertical deflecting the horizontal deflecting circuit and pre-distorting the horizontal saw-tooth deflecting waves in order to compensate for distortion produced in the deflecting coil circuit.

The amplifier tube 301 may be of the screen grid type, the plate 3| I of which is supplied with an operating voltage through a filter resistor 3I2 and a plate resistor 3|3. The filter resistor 3I2 is shunted by the usual by-pass condenser 3M.

The-saw-tooth voltage appearing in the output circuit of the amplifier tube .30l is impressed upon the input circuit of a power amplifier which, in the particular circuit being described, consisted of two power tubes 302 of the 241 type connected in parallel. Because of the high plate impedance of a tube of this type, a saw-tooth voltage component is all that is necessary in order to produce a perfect saw-tooth current through the deflecting coils 303. In other words, the impulse component, which is required when the outputtube has a low plate impedance, may be neglected in this circuit.

In order to obtain eflicient transfer of energyfrom the power tube302 to the deflecting coils 303, which have a low impedance, the output circuit of the power tube is coupled to the coils through a transformer 3|6.

The network 309 consists of two filtering and correcting stages. The first stage consists of a fllter section, comprising condenser 3H of small capacity, and a resistor 3l8, and a correcting section comprising a second resistor 3| 9 and a second condenser 31", all connected in series between the plate 304 and ground. The condenser 32l is shunted by a variable resistor 322. In operation, condenser 3I1, because of its small capacity, eliminates a large percentage of the comparatively low frequency components of the vertical heflecting saw-tooth impulses. This filtering action unavoidably removes a small percentage of the low frequency components from the horizontal deflecting saw-tooth waves, although these components have a considerably higher frequency than the components of the vertical deflecting saw-tooth waves. In order to compensate for this undesired loss, and also in order to exaggerate the amplitude and shift the phase of the low frequency components of the horizontal deflecting waves, the resistor 3H] and 'condenser32l are utilized. By adjusting the resistor 322, the low frequency components of the horizontal deflecting waves may be accentuated in the desired manner to give the necessary predistortion, ,as described in connection with the network I6l--l69--I65 shown in Fig. 1.

Because the filtering and the pre-dlstorting action of the first network stage is not perfect, the second stage is employed, this stage consisting of the small capacity condenser 323, a resistor 324, a second resistor 326 and a second condenser 321. As in the first network stage, the second condenser 32! is shunted by a variable resistor 328. The action of the second network stage is identical with that of the first stage, the condenser 323 and resistor 324 operating to remove vertical deflecting components and the resistor 326 and condenser 32! operating to increase the amplitude and' shift the phase of'the low frequency components of the horizontal deflecting waves.

It hasbeen found that the operation of the transmitter is fairly satisfactory when no attempt is made to filter out vertical deflecting components. Therefore, the specific circuit shown in Fig. 1 is the preferred one at the present time. However, a small amount of vertical deflecting components usually appears in the output of the horizontal deflecting circuit because of difliculty in balancing the circuit and, as the art progresses, it may be desired to remove this. component by means of the filter network illustrated in Fig. 13. Attention. is called to the fact that in Fig. 13 the portion of the network 309 which acts upon the low frequency components of the horizontal deflecting waves is the same as the network l6'l-|69-l65, provided for the same purpose in Fig. 1.

From the foregoing description it will be seenthat I have provided correcting circuits which enable an operator to so control the horizontal and vertical deflections of the transmitter tube electron beam that an undistorted picture is transmitted even though the transmitter tube mosaic is positioned at an angle with respect to the electron gun. It will also be seen that I have provided means either for impressing'a substantially perfect saw-tooth wave across a pair of deflect-' ing plates or for forcing a substantially perfect saw-tooth current through deflecting coils.

In the embodiment of my invention illustrated in Fig.1, certain of the circuit values were as follows: I

In the vertical deflecting circuit:

Resistor 70:4 megohms Resistor 79=400,000 ohms Resistor 77:1 megohm Resistor 8l 5,000 ohms Condenser 73:.5 micro-farads Condenser 83:.02 micro-farads Res stor 85:.5 megohms Resistor 107:.5 megohms Resistor 111:.7 ,megohms Condenser 109:.01 micro-farads In the horizontal deflecting circuit:

Transformer 183:5 to 1 step down ratio Transformer 185:15 to 1 step up ratio Other circuit values:

Resistor 201:1,000 ohms Resistor 197=25,000 ohms Resistor 199:5,000 ohms Condenser 207:4 micro-farads Condenser 205:4 micro-farads Condenser 203:.01 micro-farads It may be noted that the variable resistors are 'usually set at a value equal to about one-half the resistance values given for them in the above table.

It will be understood that various other modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and are set forth in the appended claims.

, I claim as rnv inventionz' 1. In .combination, means for producing a sawtooth voltage wave, a wave-utilization network including an amplifier tube and cathode ray deflecting means, said network-having the characteristic that it causes the more gradually sloping 'portion of a saw-tooth wave to bend over, a correcting network, and means for coupling said first means to said amplifier tube through said correcting network, said correcting network comprising resistive and reactive elements having impedance values so related that they act preferentially upon the low frequency components of said saw-tooth wave to compensate for said bending over.

2. The invention according to claim 1 characterized in that said correcting network consists of resistive and capacitive elements connected in series with each other and connected in shunt relation to the channelwhich includes said sawtooth producing means and said amplifier.

3. The invention according vto claim 1 characterized in that said correcting network consists of resistive and capacitive elements connected in series with each other and connected in shunt to the'channel which includes said saw-tooth producing means and said amplifier, and further characterized in that said capacitive element is shunted by a variable resistor.

4. In combination, means for producing a sawtooth voltage wave, a wave-utilization network including an electric discharge amplifier tube- 'saw-tooth wave to compensate for said bending over.

5. In a. cathode ray tube deflecting circuit,

means for producing a saw-tooth voltage wave, a transformer having a primary and-a secondary,

a pair of deflecting plates connected to said secondary, an amplifier tube having an input circult and having an output circuit coupled to said primary, a correcting network, and meansfor coupling said first means to said input'circuit through said network, said network including resistive and reactive elements connected in series with each other and in shunt to the channel which includes said first means and said amplifier tube.

6. The invention according to claim 5 characterized in that said reactive element is shunted by a' resistor. 1

.7. The invention according to claim 5 characterized in that said reactive elementis a condenser. I o q 8. In combination, a cathode-ray tube comprising an electron gun and a mosaic of insulated elements positioned in the path of the electron beam generated by said gun and also positioned at an acute angle to theaxis of said gun, de-

flecting means for deflecting said electron beam vertically, deflecting means iordefiecting said electron beam horizontally, means for producing a saw-tooth wave occurring at a relatively low frequency and for applying it to said first deflecting means, and means for producing a saw-tooth wave occurring at a relatively high frequency and for applying it to said second deflecting means, said last means comprising means for charging a condenser successively at a changing charging rate determined by said low frequency saw-tooth wave, an electric discharge tube, means for discharging said condenser successively through said tube at a changing discharging rate determined .by said low frequency saw-tooth wave, and a network in said last saw-tooth wave producing means which comprises resistive and reactive elements, said network being located between said condenser and said second deflecting means, said network elements being so related that they filter out said low frequency saw-tooth wave and in- -crease the amplitude of the low frequency components of said high frequency saw-tooth wave.

9. The invention according to claim 8 characterized in that said network comprises a filter section consisting o! a condenser and resistor in series and a correcting section consistingof a condenser and resistor in series, said two sections being connected in series and so connected as to have the high frequency saw-tooth wave impressed thereacross, and further characterized in that said second deflecting means is coupled across said correcting section through an amplifier tube.

' ALDA V. BEDFORD. 

